Use linux, its hard to get infected with linux. Use another web browser other than internet explorer.
Don't look up random porn crack or warez sites.
don't download a random ptp software that you know nothing about.
don't download a random bit client.
don't download things just for the sake of downloading.
don't pay for anti-virus and anti-spyware software their rip offs. get the free stuff.
If your online and somebody really smart wanted to hack into your computer then it doesnt matter what kind of protection you got he'll straight hack through all the protection you got. so don't piss anybody off in an irc chatroom or pick on smart geeky people.
Or you can keep your computer offline forever. Thats a guaranteed protection.